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Old 08-10-2010, 05:57 PM   #1 (permalink)
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Default FCD not reading correct mpg?

For two tanks now I have not been sure why the FCD seems to be reading different than my math. The second tank I actually ran my car out of gas to try to make sure I was doing the math right. Here's the first fill up:

I went 475.9 miles and filled up 11.25 Gallons (2 bars showing till complete empty) and my FCD says 53.1. Math would say 475.9/11.25=42.3.

Second fill up (where I ran the car literally out of gas) I put 2+8.666 (2 from the small tank I was carrying, and 8.66 from the station). I drove 663.1 miles and filled up 10.6. FCD reads 54.2. Math says 663.1/10.6=62.55.

Why is it so off, and in both directions? Aslo, why would my second fill up, where I was completely empty, fill up less gas than my first fill up where I had 2 bars? I'm so confused!!!:
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Old 08-10-2010, 07:00 PM   #2 (permalink)
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The tank capacity is 10.6 gallons, but you can "overfill" it by putting in so much gas that some gas goes into the evap canister. It's been posted here that one can stuff in as much as 13.5 gallons by doing this.

So I suspect you filled the two tanks to different levels: the first to a higher level than the second.

If you always fill until the nozzle clicks off once you should eliminate this problem.
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Old 08-14-2010, 10:35 AM   #3 (permalink)
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Default fcd mpg

I don't know if this helps at all... but I always fill only to auto shut off. Calculating my mpg by hand (miles/gals) is usually about 1 mpg less than the fcd calculation. So mine match up very well.
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Old 08-14-2010, 09:13 PM   #4 (permalink)
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I also fill to auto-shutoff on the pump nozzle with good results.

The FCD for me is usually off ± 3 mpg or less, compared to the calculated value.

Jim.
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Old 09-05-2010, 12:01 PM   #5 (permalink)
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This might sound odd, but it could be that one of the gas stations you used has intentionally changed the calibration on their pumps.

I know my cars well, and once I went to a gas station and filled up my Jaguar (I know, it is an insight forum, but I'm just sayin...). The pump said that I put in 19 gallons, and I know from the mark that the car should have taken about 17.5. I suspect that the station rigged the pump after it had been inspected. Every time before when I filled up the car at that mark, the car had taken 17.5. I have had other times when I felt like a car took more fuel than the guage indicated.

If you filled up at a station that is not 100% trusted, it could happen.

Most states have programs that require pumps be certified, but I am convinced that cheating at the pumps happens. Most often I think it happens at non-corporate owned convenience stores. I tend to use corporate owned stations because I believe that the big corporations are far to smart to get caught rigging the pumps.

On a small tank, the innacuracy may not show, but as the tank gets larger, I think it starts to show more. Just my opinion, but I am fairly certian that this practic occurs. My advice is to use corporate owned stations. I always feel like the amount pumped in matches the amount I estimate.

YMMV.. Literally... LOL

Just something to consider.

Last edited by eddgie; 09-05-2010 at 12:07 PM.
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Old 09-05-2010, 02:50 PM   #6 (permalink)
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My bet is you are overfilling it compared to the previous fill up. Personally I let it click once then do one more squeeze every time and find mine is with 1- 2% of calculated.
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Old 09-06-2010, 08:05 AM   #7 (permalink)
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Quote:
This might sound odd, but it could be that one of the gas stations you used has intentionally changed the calibration on their pumps... but I am fairly certian [sic] that this practic occurs.
Seriously. I don't see it happening. Back in my insurance days (commercial lines underwriting supervisor), we had agencies that covered gas stations, so I know first-hand all the hoops they have to jump through just to open the doors, meet underwriting requirements, and stay in business in a most-competitive environment. Too many security benchmarks for them to meet before they can sell fuel... too much downside for the retailer if they try to defeat these benchmarks... too many repercussions for the owner/manager when caught. And they will be caught. They always are.

But if you want to ease your worried mind, calibrate a five-gallon jerry can, go fill it up at the allegedly-offending station, and judge then. If the volume is off, do it again, this time inviting the hapless manager and the local TV stations to watch and film as you repeat the experiment.

And as you may know, there is another variable to consider: The volume of the pumped fuel varies slightly according to temperature. Flowmeters are not temperature-compensated in the US (I believe Canada's are, if memory serves)... Filling up early in the day, when storage tank temperatures are slightly cooler, yields slightly more density of gasoline than filling up later in the day, when summer temperatures have soared and the fuel has expanded by volume. In other words: A gallon of cooler gasoline will occupy less volume than a gallon of warmer fuel. Fill up when fuel is cooler, and get marginally-more fuel into your tank. The per-gallon pump won't account for that small, but measurable, difference.

But my take is, you're just whipping up an urban legend, not reporting on actual facts. Deliberate, underpumping shenanigans... It just ain't happening...
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Old 09-06-2010, 12:14 PM   #8 (permalink)
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My advice would be fill at the same station and same pump (if you can) for multiple tanks and stop when the pump auto-stops. Even take note of the ambient air temp when you fill, and try to do it when it is around the same temp and time of day.

I did this for several tanks and found my FCD to be optimistic (eg calculated numbers were worse MPG) by ~1%. Based on others statements here it seems like they can vary by vehicle. FWIW, my HCH is 3-4% optimistic.

I have never heard of one being off by 5-10 MPG like you report. I would think it has to do with how you fill, topping off and filling the evap canister as suggested previously.
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Old 09-08-2010, 05:03 PM   #9 (permalink)
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Originally Posted by lsixecho View Post
For two tanks now I have not been sure why the FCD seems to be reading different than my math. The second tank I actually ran my car out of gas to try to make sure I was doing the math right. Here's the first fill up:

I went 475.9 miles and filled up 11.25 Gallons (2 bars showing till complete empty) and my FCD says 53.1. Math would say 475.9/11.25=42.3.

Second fill up (where I ran the car literally out of gas) I put 2+8.666 (2 from the small tank I was carrying, and 8.66 from the station). I drove 663.1 miles and filled up 10.6. FCD reads 54.2. Math says 663.1/10.6=62.55.

Why is it so off, and in both directions? Aslo, why would my second fill up, where I was completely empty, fill up less gas than my first fill up where I had 2 bars? I'm so confused!!!:

If you put the two tanks together, you come pretty close.

I didn't bother weighting it, but you drove 1139 miles on 21.85 gallons. That comes out to 52.1, which seems about midway between your two averages and close to both FCD calculations. I would say your FCD is pretty much within the margin of error (2-3%).

Try filling to the auto shutoff, round out to the nearest 5 or 10 cents ( I generally round to the nearest 25 cents) and then do a couple of more tank calculations. You should be OK.
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Old 09-10-2010, 03:29 PM   #10 (permalink)
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You may also find that your odomoter is off by a few percentage points.

On my car, my actual speed is about 2-3% lower than what it states (based on what my GPS calculates as well as the occasional roadside radar sign).

This results in my calculated MPG also being 2-3% lower than what the FCD and lifetime/trip MPG state.
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